Carbon nanostructure of the present invention is a nanosize material composed of carbon atoms. For example, there are carbon nanotube, beaded carbon nanotube in which beads are formed on a carbon nanotube, carbon nanobrush in which many carbon nanotubes bristle, carbon nanotwist in which a carbon nanotube is twisted, carbon nanocoil in a coil form, and carbon nanohorn. In the following, these various carbon substances are referred to as carbon nanostructures.
As for a method to produce these carbon nanostructures, there exists a thermal CVD method, a type of chemical vapor deposition method (CVD, Chemical Vapor Deposition) where a raw material gas of hydrocarbon is decomposed so that a given material is grown. That is to say, a catalyst chemical vapor deposition method (CCVD method, Catalyst Chemical Vapor Deposition), in which a given material is grown using a catalyst, is known.
The present invention relates to a method for producing carbon nanostructures by said thermal CVD method. This chemical vapor deposition method refers in general to a method for decomposing a raw material gas in a reaction container, so that a given material is grown. Various decomposing means such as heat, electron beam, laser beam, ion beam and such are included among the decomposing means.
Conventionally, to produce carbon nanostructures by chemical vapor deposition, a production method is adopted, in which a mixed gas of a raw material gas and a carrier gas is introduced into a reaction chamber, the raw material gas is decomposed by a catalyst, and the carbon nanostructures are grown on the catalyst surface.
For example, when carbon nanostructures are applied to an electron emission source, an electronic part of an electrode and such, or a fuel cell, the physical property of the carbon nanostructures influences its performance. In particular, the uniformity of the physical properties such as diameter, density, height (film thickness) and such is required. In addition, the uniformity of carbon nanostructures obtained by a thermal CVD method depends greatly upon the formation precision of the catalyst layer used for the growth.
In addition, for these applications, it is desirable for the carbon nanostructures to be highly oriented. The high orientation, as referred here, signifies that the direction of the carbon nanostructures grown from a substrate is even. This is also referred to as brush-like. The direction of a carbon nanostructure usually is perpendicular with respect to the substrate, and the lengths of carbon nanostructures are approximately equal (see FIG. 8). Such highly oriented carbon nanostructures are used with the whole substrate, or after being transferred while maintaining the highly oriented state. Applications for such highly oriented carbon nanostructures are being developed as a capacitor, an electrode, and in production of a rope that is spun directly from the substrate. On the other hand, it is also used as a mass production method of carbon nanostructures with even lengths.
Formation methods of catalyst layers used in a thermal CVD method are disclosed in Patent Document 1-4. In Patent Document 1, the formation of a catalyst layer through evaporation by electron beam is described In Patent Document 2, the formation of a catalyst coating film layer is described, in which a catalyst metal compound is combined with an alcohol compound and an amino compound and then converted into a paste, and this paste is coated on a substrate. In Patent Document 3, the formation of a catalyst coating film layer is described, in which iron nitrate is dissolved in a liquid mixture of an alcohol, such as ethanol, and terpineol, and then this solution is coated on a substrate of a silicon wafer. Patent Document 4 describes the adhesive formation of fine particles of a catalyst metal oxide by dissolving ferric acetate in a solvent comprising alcohols such as ethanol, dip-coating or spreading this solution on a silicon substrate, heat-treating in an oxidative atmosphere, removing of the solvent component remaining on the substrate by oxidative degradation, and heat-treating in the atmosphere of an inert gas or a gas having a reductive property.    [patent document 1] Japanese Patent Laid-Open No. 2005-75725    [patent document 2] Japanese Patent Laid-Open No. 2007-91530    [patent document 3] Japanese Patent Laid-Open No. 2006-239618    [patent document 4] PCT WO2004/071654 Bulletin